1. Field of the Invention
The present invention generally relates to a method of crystallizing/activating a polysilicon layer and a method of fabricating a thin film transistor having the same polysilicon layer, and more particularly, to a method of crystallizing/activating a polysilicon layer and a method of fabricating a thin film transistor having the same polysilicon layer, in which a channel range of an amorphous silicon layer is crystallized and at the same time source/drain regions thereof is activated.
2. Discussion of Related Art
Amorphous silicon used as an active layer of a thin film transistor is advantageously fabricated by a simple process and at low temperature, but its electron mobility is relatively low, so that it is unsuitable to a high-speed circuit. Oppositely, in the case where polysilicon is used as the active layer, the electron mobility thereof is relatively high, so that it is suitable to the high-speed circuit even though its fabricating process is complicated. This is because the polysilicon has a finer crystallization structure and fewer defects than the amorphous silicon.
As a method of crystallizing the polysilicon, there are a solid phase crystallization (SPC) method, a metal induced crystallization (MIC) method, an excimer laser annealing (ELA) method, a static light scattering (SLS) method, etc. The SPC crystallizes the amorphous silicon at a high temperature, so that the SPC has an advantage of good layer quality but has a disadvantage of requiring a high temperature process. The MIC heats a predetermined metal deposited on the amorphous silicon and crystallizes the amorphous silicon, wherein the metal is employed in lowering an enthalpy of the amorphous silicon. The MIC is performed at a low temperature but has disadvantages that surface quality of the crystallized amorphous silicon is bad and its characteristic is deteriorated due to the metal.
The ELA and the SLS melt the amorphous silicon by a laser in an instant (e.g., ˜30 nsec) and recrystallize it. The ELA melts the amorphous silicon by instantly supplying laser energy to the substrate on which the amorphous silicon is deposited, and refrigerates it, thereby crystallizing the polysilicon based on a silicon seed. Here, the silicon seed is some of the amorphous silicon, which is not melted by the laser. Oppositely, the SLS partially melts the amorphous silicon exposed to a laser beam through a mask, and employs the other portion of the amorphous silicon, which is not exposed to the laser beam, as the silicon seed. Here, a grain is vertically grown in a boundary between a liquid silicon region and a solid silicon region, but its lateral growth is also possible by adjusting power and irradiated region of the laser energy.
Hereinbelow, the conventional method of fabricating the polysilicon thin film transistor will be described with reference to FIGS. 1A through 1D which are cross sectional views illustrating a process of fabricating a conventional polysilicon thin film transistor.
Referring to FIG. 1A, a buffer layer 2 and an amorphous silicon layer are deposited on a substrate 1 in sequence. Then, the amorphous silicon is crystallized into the polysilicon by a laser 4 to be well-absorbed in the amorphous silicon.
Referring to FIG. 1B, a gate dielectric layer 5 and a gate material 6 are deposited, wherein the gate material 6 has good reflexibility. Then, a gate is formed by a photolithography process, and only a polysilicon layer of source/drain regions is doped by an ion implantation process.
Referring to FIG. 1C, a laser beam is irradiated to both the gate material and the polysilicon layer, so that the gate material having the good reflexibility is nearly invariable in temperature but the dopant of the polysilicon layer well-absorbing the laser beam is activated.
Referring to FIG. 1D, an interlayer dielectric layer 9 is deposited and then formed with a contact hole. Then, a metal layer 10 is formed.
Thus, in the conventional method, the crystallizing process and the activating process are separately performed, thereby complicating the process and increasing a processing cost.